1. Field of the Invention
This invention relates to an optoelectric detector having light emitting diodes (LEDs) separated from a photoconductor by a wall so that light from the LEDs must pass out of the detector and into a slurry to be reflected back to the photoconductor.
2. Background of the Invention
In commonly owned U.S. Pat. No. 4,950,908 there is disclosed a detector used to optimize the operation of a coal slurry processing cell. This detector has LEDs separated from a photoconductor by an opaque barrier which extends to the surface of the detector housing.
In using this detector to measure solids concentration in a coal slurry, it was discovered the glass window of the detector conducted light around the barrier and illuminated the photoconductor to a small but finite degree. This limited the extent of solids concentration measurement to about 4 wt % depending on the blackness or ash content of the solids.
Other devices such as turbidity meters are used to determine the solids concentration in a slurry. Most commercially available turbidity meters comprise two windows, usually parallel to each other, between which slurry is placed. Light passes from the source through its protective window and through the slurry. The particles in the slurry absorb or scatter some of the light and attenuate the beam which arrives at the photosensor. In order to measure slurries which contain black solids like coal above a concentration of about 2 wt %, the light must be made very bright and the path length through which the light passes must be only a few thousandths of an inch long. The brightness of the beam is limited by the size and temperature that can be tolerated in the instrument, and the transmittance path shortness is limited by particle size and flow characteristics of the slurry. Attempts to make a transmittance meter that could perform in slurries containing coals in concentrations above 2 wt % failed. The readings of the slurry in the narrow gap through which the slurry passes were unstable, and without doubt, the detector would have to be cleaned often or become plugged with solids.
The same components can be arranged with the windows placed at right angles. Light strikes the particles in the slurry and is scattered through the photosensor window and is detected. This type of meter is called a nephelometer. It is often used for measuring the cloudiness of clarified water, hence extremely dilute suspensions. Other attempts have been made to completely isolate the photoconductor from the LEDs to obtain a more accurate reading of solids concentration in a medium. U.S. Pat. Nos. 3,185,975, 3,263,553 and 3,861,802 illustrate placing barriers between the light source and light sensor. US Patents illustrate other types of detectors.